re: Is the mini-series best for great storytelling?

Yes and no.

Yes: a story should have a main arc with a beginning middle and end, and it should not be dragged on until it dies before it reaches the climax and denouement. This means that a mini-series is a better form of story telling because it necessarily contemplates a beginning, middle and end without the TV executives needing the writers to come up with more filler. The story is told with the understanding that one season or a given amount of episodes will be done, and the characters grow and fall in the manner that is most organic to the story.

No: just because that principle applies most readily to mini-series does not mean that it cannot apply to a series. I think the best examples of this are actually found in anime. Anime series like Wolf's Rain know where they will begin and end from the time they are conceived. 2 seasons is what was needed to tell the story, so they did 2 seasons. Very rarely do you see that in American television, but Breaking Bad was very close.